Grass Hedge Effects on the Transport of
Phosphorus, Nitrogen and Sediment Following
Field Application of Beef Cattle Feedlot Manure

Bahman. Eghball
Agronomy Department, University of Nebraska, Lincoln

John E. Gilley
USDA-ARS, University of Nebraska, Lincoln

Larry A. Kramer
USDA-ARS, Deep Loess Research Station, Council Bluffs, Iowa

Tom B. Moorman
Microbiologist, USDA-ARS, National Soil Tilth Laboratory, Ames, Iowa

Abstract

Runoff losses of nitrogen (N), phosphorus (P), and sediment from field-applied manure can contribute to surface water pollution. Grass hedges may reduce runoff losses of nutrient and sediment. The objective of this study was to evaluate the effects of narrow grass hedges (0.75 m wide) on the transport of P, N, and sediment from a field receiving beef cattle feedlot manure in disked and no-till systems. This study was conducted on a steep (12 % slope) Monona soil near Treynor, Iowa. The experiment was a split-plot with no-till and disked systems as main plots and subplots of manure, fertilizer and check with or without a grass hedge. A rainfall simulator was used and runoff was collected from both the initial and wet runs. Results indicate that there was no effect of tillage on runoff concentration of dissolved P (DP), bioavailable P (BAP), total P or nitrate during both runs. Grass hedges reduced runoff concentration of DP and BAP from manure application during the wet run by 48% each in the no-till plots and by 13% and 25% in the tilled plots, respectively, as compared with no hedge. Runoff nitrate concentration from fertilizer applied to the tilled plots was reduced by 21% during the wet run when grass hedges were used. Grass hedges reduced runoff nitrate concentration from manured plots by 28% during the initial run as compared with no hedge. The amounts of runoff and erosion were less for no-till than for the disked system during both runs. Grass hedges significantly reduced erosion and runoff during the wet run. Narrow grass hedges were effective in reducing soil erosion, and P and nitrate losses in runoff from manure and fertilizer application.

Environmental concerns must be addressed on areas receiving manure. Runoff from cropland areas receiving manure may contribute to increased phosphorus and nitrogen concentrations in streams and lakes. Even though P from manure application may move deep into the soil (Eghball et al, 1997), the primary concern about P pollution is with eutrophication of surface waters. The main factors controlling P movement in surface runoff are transport (runoff and erosion) and source factors (manure or fertilizer application and soil P test level) (Sharpley et al., 1993). Manure application less than 15 Mg ha^-1 does not seem to contribute to P or N enrichment of surface waters (Jones and Willis, 1995).

Filter strips have been shown to substantially reduce nutrients and sediment in runoff from cultivated agricultural areas (Dillaha et al., 1989). Recently, narrow grass hedges, planted on the contour along the hillslope, have been used as an effective conservation practice. Little work has been conducted to evaluate the performance of grass hedges for controlling P and N movement, and erosion control. The objective of this study was to determine the effects of narrow switchgrass hedges on the transport of P, N, and solids flowing a single application of manure under no-till and tilled conditions.

This study was conducted at the USDA-ARS Deep Loess Research Station approximately 19 km east of Council Bluffs, Iowa. A Monona soil was used in the investigation.

The study site had been in continuous corn since 1964 and had been managed using spring tillage for seed bed preparation and weed control. In May 1991, switchgrass hedges approximately 0.75 m wide were established within a 6 hectare watershed. The grass hedges were separated by 16 corn rows.

This study was conducted as a split plot each with three replications (36 total plots). Main plots consisted of no-till and tilled conditions and subplots included manure (58.6 Mg dry weight ha^-1), inorganic fertilizer (151 kg N ha^-1 and 25.8 kg P ha^-1) and no treatment check plots. For each treatment (6 total) runoff was collected either above or below the grass hedge. The manure and fertilizer were applied by hand at the approximate rates required to meet N requirements for a corn crop with a target yield of 9.4 Mg ha^-1. The manure used in this study had a total N content of 0.82% and a total P content of 0.55% on dry weight basis. It was assumed that the plant N availability from manure during the year of application was 40%.

A portable rainfall simulator was used to apply rainfall simultaneously to two 3.7 m wide by 10.7 m long plots which were established using sheet metal borders. An initial one-hour rainfall application at an intensity of approximately 6.4 cm hr^-1 occurred at existing soil-water conditions. A second one-hour application (wet run) was conducted approximately 24 h later. The plots were covered with plastic sheets between the initial and wet runs to eliminate the input of natural rainfall into the system.

A trough extending across the bottom of each plot gathered runoff, which was measured using a flume with stage recorder. Runoff samples were collected in plastic bottles at 5-min intervals from each trough. The runoff samples from 5, 10, 15, 30, 45, and 60 minutes after runoff initiation were analyzed for dissolved P (DP), bioavailable P (BAP), total P, nitrate, ammonium, total N, EC and pH. Because of the space constraints, only DP, BAP, total P and nitrate is reported in this paper. Bioavailable P is the P fraction in runoff that is available to algae (Sharpley, 1993). Additional runoff samples were later placed in an oven maintained at a temperature of approximately 106° C for erosion measurement. Colored slides were taken at three locations on each plot, and by projecting the slide on a grid, surface cover was determined.

Analysis of variance indicated significant tillage by treatment interactions for all the nutrients reported. Therefore the values for tillage and treatments are reported in Table 1 for both simulation rainfall runs. Grass hedge significantly reduced runoff concentration of dissolved P, bioavailable P and total P from applied manure in both tillage systems (Table 1). However, grass hedges had no effect on runoff concentration of these P parameters from applied P and N fertilizers (Table 1). Grass hedges significantly decreased runoff concentration of nitrate in the tilled plots receiving ammonium nitrate fertilizer but had no effect on nitrate concentration in the no-till during the wet run. Grass hedge also reduced runoff concentration of nitrate from tilled manure plots during the initial run (Table 1). Manure application resulted in greater runoff DP and BAP concentration than fertilizer application during both runs. Total P in runoff from manured plots were similar to those from fertilized plots but both had higher concentration than the check plots (Table 1).

The mean value of the slope gradient at the study site was 12%. Residue cover ranged from 51% to 94% on the no-till treatments, and 11% to 58% on the tilled plots. Mean residue covers for the no-till and tilled treatments were 79% and 34%, respectively. Thus, the single disking operation caused mean corn residue cover to be reduced by 57%. The application of manure (58.6 Mg ha^-1) to meet corn N requirements on the no-till plots resulted in 22% reduction in residue cover as compared to the check plots. No effect of manure application on residue cover was observed in the disked plots.

The grass hedges reduced runoff by 57% and 40% on the no-till and tilled plots, respectively (Table 2). For both no-till and tilled conditions, total runoff was similar on the check and manure treatments located above the grass hedge (Table 2). Thus, the application of manure at a rate necessary to meet corn production requirements did not significantly affect total runoff.

For both the initial and wet rainfall simulation runs, tillage of the soil surface resulted in a significant increase in solids yield (Table 2). Solids yield can be substantially impacted by the reduction in surface cover caused by tillage. Solids yield measurements above the grass hedges were larger than values obtained from plots with the runoff collection unit located below the grass hedges (Table 2). When total solids yield in the check plots are considered, the grass hedges reduced solids yield by 45% and 76% on the no-till and tilled plots, respectively. However, these differences in solid yield measurements were not statistically significant. Variations in the amount of concentrated flow entering the hedges may have caused substantial variations in solids delivery between replicates of a given experimental treatment.

Narrow grass hedges (~0.75 m) were effective in reducing erosion, and runoff concentration of dissolved P, bioavailable P, total P and nitrate. They can be used as an effective conservation method for steep soils.

Table 1. Runoff concentration of nutrients as affected by tillage and fertility treatments.

	Initial Run				Wet Run Treatment†
	DP‡	BAP	Total P	Nitrate	DP	BAP	Total P	Nitrate
No-till	---------------------------------------- mg L-1 ----------------------------------------
Check BH	0.32 bc*	0.73 b	3.0 c	20.5 b	0.28 d	0.88 d	3.4 c	23.8 c
Check AH	0.17 c	0.75 b	4.9 b	19.9 b	0.14 d	0.82 d	3.4 c	20.0 d
Fertilizer BH	1.18 abc	2.10 a	8.2 a	32.3 a	0.76 c	1.19 d	4.6 bc	31.2 a
Fertilizer AH	1.80 a	2.96 a	8.5 a	33.8 a	0.98 c	1.62 c	5.7 bc	30.6 a
Manure BH**	-	-	-	-	1.61 b	2.31 b	6.5 b	26.0 b
Manure AH	1.63 ab	2.90 a	8.4 a	29.5 a	3.11 a	4.48 a	9.8 a	27.3 b
Tillage
Check BH	0.45 bc	0.72 c	2.3 d	23.1 c	0.28 c	0.90 c	2.5 d	19.6 c
Check AH	0.11 d	0.51 c	5.7 bc	23.5 c	0.10 d	0.82 c	4.8 bc	18.5 c
Fertilizer BH**	-	-	-	-	0.31 c	0.85 c	4.2 bc	23.9 b
Fertilizer AH	0.35 c	1.15 b	7.5 b	45.6 a	0.27 c	0.80 c	5.1 ab	30.2 a
Manure BH	0.56 b	1.19 b	4.8 c	27.4 c	1.12 b	1.66 b	3.9 c	25.6 b
Manure AH	1.05 a	1.98 a	5.8 bc	38.0 b	1.29 a	2.20 a	6.2 a	26.6 b
† AH (runoff collected above hedge), BH (runoff collected below hedge) ‡ DP is dissolved P and BAP is bioavailable P. * Within each run and tillage systems and for each column, differences are significant at the 10% level (Duncan's test) if the same letter does not appear. ** No runoff in the initial run.

Table 2. Runoff and solids yield from the initial and wet rainfall simulation runs on the no-till plots^*.

Treatment†	Initial Run		Wet Run
	Runoff	Solids yield	Runoff	Solids yield
No-till	mm	Mg/ha	mm	Mg/ha
Check BH	0.2 a‡	0.01 b	6.6 c	0.39 ab
Check AH	0.6 a	0.03 ab	15.1 a	0.70 a
Fertilizer BH	0.2 a	0.01 b	7.5 bc	0.30 ab
Fertilizer AH	0.5 a	0.07 a	13.6 ab	0.70 a
Manure BH	0.0 a	0.00 b	4.7 c	0.27 b
Manure AH	0.1 a	0.01 b	10.3 abc	0.57 ab
Tillage
Check BH	2.4 c	0.08 b	13.6 b	0.45 c
Check AH	3.7 c	0.42 b	22.8 ab	1.75 c
Fertilizer BH	2.0 c	0.26 b	21.8 ab	1.25 c
Fertilizer AH	4.4 c	0.72 b	21.7 ab	2.06 c
Manure BH	1.7 c	0.15 b	15.2 b	0.54 c
Manure AH	1.8 c	0.15 b	18.4 b	1.20 c
Residue Removed BH	11.5 b	2.37 b	28.8 ab	8.48 b
Residue Removed AH	30.5 a	15.31 a	37.4 a	14.09 a
* Values given are the average of three replications. Runs lasted for a 60 minute duration at 64 mm/hr. † AH (runoff collected above hedge), BH (runoff collected below hedge) ‡ Within each run and column, differences are significant at the 10% level if the same letter does not appear based on Duncan's Multiple Range Test. Metric to English unit conversion: 25.4 mm = 1 in; 2.24 Mg/ha = 1 ton/acre.

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